Force Methods/Flexibility Method Study Notes for Civil Engineering

This article contains fundamental notes on "Force Methods/Flexibility Method" topic of "Structure Analysis" subject. Also useful for the preparation of various upcoming exams like GATE Civil Engineering(CE)/ IES/ BARC/SSC-JE /State Engineering Services examinations and other important upcoming competitive exams.

Force Methods/Flexibility Method

Introduction

For determinate structures, the force method allows us to find internal forces (using equilibrium i.e. based on Statics) irrespective of the material information. Material (stress-strain) relationships are needed only to calculate deflections.

However, for indeterminate structures , Statics (equilibrium) alone is not sufficient to conduct structural analysis. Compatibility and material information are essential.

The flexibility method is based upon the solution of "equilibrium equations and compatibility equations". There will always be as many compatibility equations as redundants. It is called the flexibility method because flexibilities appear in the equations of compatibility. Another name for the method is the force method because forces are the unknown quantities in equations of compatibility.

The force method is used to calculate the response of statically indeterminate structures to loads and/or imposed deformations. The method is based on transforming a given structure into a statically determinate primary system and calculating the magnitude of statically redundant forces required to restore the geometric boundary conditions of the original structure.

The basic steps in the force method are as follows:(a) Determine the degree of static indeterminacy, n of the structure.(b) Transform the structure into a statically determinate system by releasing a number of static constraints equal to the degree of static indeterminacy, n. This is accomplished by releasing external support conditions or by creating internal hinges. The system thus formed is called the basic determinate structure.(c) For a given released constraint j, introduce an unknown redundant force Rf corresponding to the type and direction of the released constraint.(d) Apply the given loading or imposed deformation to the basic determinate structure. Use suitable method (given in Chapter 4) to calculate displacements at each of the released constraints in the basic determinate structure.(e) Solve for redundant forces Rf ( j =1 to n ) by imposing the compatibility conditions of the original structure. These conditions transform the basic determinate structure back to the original structure by finding the combination of redundant forces that make displacement at each of the released constraints equal to zero.

Types of Force Method/Flexibility Method/Compatibility Methods

Virtual work/Unit load Method

Method of consistent deformation

Three-moment theorem

Castigliano's theorem of minimium strain energy

Maxwell-Mohr's equation

Column Analogy Method

Strain Energy Method

Strain energy stored due to axial load

where, P = Axial load

dx = Elemental length

AE = Axial rigidity

Strain energy stored due to bending

where, MX = Bending moment at section x-x

ds = Elemental length

El = Flexural rigidity

or E = Modulus of elasticity

l = Moment of inertia

Strain energy stored due to shear

where, q = Shear stress

G = Modulus of rigidity

dv = Elemental volume

Strain energy stored due to shear force

where, AS = Area of shear

S = Shear force

G = Modulus of rigidity

ds = Elemental length

Strain energy stored due to torsion

where, T = Torque acting on circular bar

dx = Elemental length

G = Modulus of rigidity

lP = Polar moment of inertia

Strain energy stored in terms of maximum shear stress

where, Maximum shear stress at the surface of rod under twisting.

G = Modulus of rigidity

V = Volume

Strain energy stored in hollow circular shaft is,

Where, D = External dia of hollow circular shafts

d = Internal dia of hollow circular shaft

Maximum shear stress

Castigliano's first Theorem

where, U = Total strain energy

Δ = Displacement in the direction of force P.

θ = Rotation in the direction of moment M.

Castiglianos Second Theorem

Betti's Law

where, Pm = Load applied in the direction m.

Pn = Load applied in the direction n.

δmn = Deflection in the direction 'm' due to load applied in the direction 'n'.

δnm = Deflection in the direction 'n' due to load applied in the direction 'm'.

Maxwells Reciprocal Theorem

δ21 = δ12

where, δ21 = deflection in the direction (2) due to applied load in the direction (1).

δ12 = Deflection in the direction (1) due to applied load in the direction (2).

The degree of static indeterminacy = 3–2 =1. The moment at B is taken as redundant R and the basic determinate structure will be then two simply supported beams as shown below

The bending moment diagram of the beam is shown in Figure 4.

Example 2

Find the force in various members of the pin-jointed frame shown in figure below. AE is constant for all members.

Sol:

The static indeterminacy of the pin-jointed frame = 1. The vertical reaction at C is taken as unknown force R . The computation of deflection of point C due to applied loading and R are shown in Tables 2 and 3, respectively.

Example 3

Find the expression for the prop reaction in the propped cantilever beam shown in the figure below.

Sol:

Let reaction at support A be R . According to the Castigliano's theorem

Example 4

Analyze the continuous beam shown in figure shown beolw by the three moment equation. Draw the shear force and bending moment diagram.

Sol:

The simply supported bending moment diagram on AB and AC are shown below. Since supports A and C are simply supported